1. Field of the Invention
The present invention relates generally to systems and method for extracting gases from fluid mixtures. More particularly, the invention relates to the electrochemical extraction of oxygen employing electrocatalysts which promote oxygen release by facilitating electron transfer to and from carrier compounds which reversibly bind the oxygen.
Purified gases, such as oxygen, are useful in a number of industrial, scientific and medical applications. Such gases may be obtained in a variety of ways. Large-scale extraction of gases from air may be accomplished by cryogenic fractionalization where the air is liquified and separated based on the differing boiling points of its constituent gases. Although practical for producing large volumes of gases, cryogenic fractionalization is impractical for supplying small gas volumes, particularly at remote or inaccessible locations.
As an alternative to cryogenic fractionalization, a variety of small-scale techniques have been developed for producing relatively pure gases. For example, hydrogen and oxygen may be produced by the electrolysis of water under controlled conditions. Although practical for many applications, electrolysis suffers from relatively high energy requirements and a substantial danger of explosion resulting from the presence of molecular hydrogen. Methods have also been developed for extracting dissolved gases from liquids, particularly fresh sea water. Such methods generally employ gas-permeable membrane for extracting the gases. As with electrolysis, membrane gas extraction is useful, but suffers from a number of limitations. In particular, most membranes are nonselective and will pass whatever gases are dissolved in the sea water. Moreover, the pressure of the collected gas generally cannot exceed the partial pressure in the sea water, at least in the absence of suitable compression and storage equipment.
Recently, systems have been developed for extracting oxygen from fluid mixture feedstocks based on the use of transition metal coordination compounds which in a first oxidation state bind the oxygen molecules and in a second oxidation state release the oxygen molecules. The systems, as described in U.S. Pat. Nos. 4,602,987, 4,609,383, and 4,629,544 rely on circulating the carrier compounds past a first location where the oxygen is bound, typically through an oxygen-permeable membrane. The oxygen-loaded carrier compounds are circulated past a first electrode where their oxidation state is changed, causing release of the oxygen which may be then collected and stored or utilized. The unloaded carrier compounds are then circulated past the second electrode of the electrochemical cell, where they are returned to their first oxidation state. The carrier compounds are then returned to the loading station where they can again bind oxygen from the fluid mixture.
Such systems have several advantages. First, the energy requirement is low relative to other extraction techniques, particularly electrolytic decomposition of water. Second, the partial pressure of oxygen which may be obtained is limited only by the solubility of the organometallic carriers in the circulating carrier fluid. Thus, oxygen pressures which are much higher than the partial pressure in the fluid mixture may be obtained without use of supplemental compression equipment.
Despite the substantial advance represented by U.S. Pat. Nos. 4,602,987, 4,609,383, and 4,629,544, it would still be desirable to provide improvements in the systems described. In particular, it would be desirable to enhance oxygen extraction by promoting the transfer of electrons from the carrier compounds to the anodic electrode and/or from the cathodic electrode to the carrier compounds. Such promotion would increase the volume output of oxygen from a fixed sized cell or, alternatively, allow a fixed amount of oxygen to be produced by a cell having reduced electrode area and/or lower power consumption.
2. Description of the Background Art
Various electrochemically active transition metal complexes and organic substances can be bound to or incorporated in electrode surfaces. For example, Doblhofer and Durr, J. Electrochem. Soc. (1980) 127:1041, disclose codeposition of acrylonitrile monomers with metal acetylacetonates on glassy carbon electrodes to form an electrode surface for cathodic reduction of molecular oxygen. Dubois, et al., J. Electroanal. Chem. (1981) 117:233, disclose ferrocene derivatives covalently linked to polytyramine films and their electrochemical properties. Oyama and Anson, J. Electrochem. Soc. (1980) 127:640, disclose the electrochemical characteristics of ruthenium complexes bound to poly-(4-vinylpyridine) coated electrodes. Naphthaquinones have been bound to an electrode surface and used to catalyze the reduction of O.sub.2 to H.sub.2 O.sub.2 Calabrese, et al., J. Am. Chem. Soc. (1983) 105:5594-5600. Methods of preparing chemically modified electrodes are reviewed by Murray, Acc. Chem. Res. (1980) 13:135.
A number of surface groups have been identified on carbon electrodes, including carboxyl, phenolic hydroxyl, quinone, normal lactone, fluorescein-like lactone, carboxylic anhydride, and cyclic peroxide (Comprehensive Treatise of Electrochemistry, Vol. 4, Chap. 10, p. 479); cyclic esters (Barton and Harrison, Carbon (1975) 13:283-288); quinoid, quihydrone, phenolic, carboxyl, carbonyl, lactone, sulfates, metal acid derivatives, and lamellar compounds (Panzer, Electrochimica Acta (1975) 20:635-647); (carboxyl, alcoholic, phenolic hydroxyl, quinones, and lactones (Bensenhard and Fritz, Angew. Chem. Int. Ed. Engl. (1983) 22:950-975); and ketone carbonyl groups (Mattson and Mark, Jr., J. Coll. Inter. Sci. (1969) 31:131-144).
The surfaces of carbon electrodes have been modified by a number of techniques, including electrochemical oxidation (Weinberg and Reddy J. App. Electrochem. (1973) 3:73-75; Vasquez and Imai, Bioelectrochem. Bioener. (1985) 14:389-403; and U.S. Pat. No. 3,657,082); oxidation in a heated airstream (Hollax and Cheng, Carbon (1985) 23:655-664); voltage cycling with a planar electrode surface Engstrom, Anal. Chem. (1982) 54:2310-2314; Blaedel and Schieffer, J. Electroanal. Chem. (1977) 80:259-271); and acid oxidation of carbon fibers (Proctor and Sherwood, Carbon (1983) 21:53-59). Various pretreatments have been reported to increase electron transfer rates to certain couples (Engstrom, supra, and Blaedel and Schieffer, supra).
The mechanism of electrochemical catalysts in promoting the reduction of carbon-halogen bonds is described in Andrieux et al , J. Am. Chem. Soc. (1979) 101:3431-3441, and (1984) 106:1957-1962, and Saveant and Binh, J. Electroanal. Chem. (1978) 88:27-41.